Off-grid generator sets have been widely used. For example, a portable/standby generator set may be utilized to power electrical equipment connected via a circuit with the generator set when people do camping, tailgating and working outside. A similar power generating apparatus provides backup power in emergencies, for example during blackout.
The off-grid generator set usually utilizes a driving engine coupled to a generator (or alternator) through a common shaft. Upon actuation of the engine, the engine rotates the common shaft to drive the alternator that, in turn, generates electrical power. As it is known, most residential electric equipment is designed to be used in connection with electrical power having a fixed frequency, for example, 60 hertz (Hz) in North America and 50 Hz in China. The frequency of the output electrical power is primarily determined by the operating speed of the engine. As a result, the engine's operating speed of some generator sets is fixed as so to keep the frequency of the output electrical power fixed. However, when an electrical load applied to the generator set is less than the rated kilowatt load for which the generator set is designed, the engine's fuel-efficiency deteriorates and the generator set generates loud noise. Additionally, the off-grid generator set usually generates an output voltage at a known level since most electrical loads are designed to work with a voltage of the known level. For example, most household appliances in North America such as ovens, roasters, audio and video displayers use nominal voltages of 120 volts or 240 volts, and most household appliances in China use a nominal voltage of 220 volts.
It is therefore desirable to have a power generating apparatus that allows a variable engine speed, an output voltage with a constant amplitude and a constant frequency.